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Concurrency related issues manifest themselves when assumptions are made about the multithreaded behavior of derived classes. An overridden synchronized method's contract may be violated if a subclass provides an implementation that is not safe for concurrent use.

The guideline CON04-J. Synchronize Use private final lock objects to synchronize classes that may interact with untrusted code using a private final lock object recommends documenting the locking strategy of classes designed for inheritance. This information is useful for deciding the locking strategy of subclasses.

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This compliant solution does not violate CON04-J. Synchronize Use private final lock objects to synchronize classes that may interact with untrusted code using a private final lock object because the accessibility of the class is package-private which is allowable when untrusted code cannot infiltrate the package.

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This noncompliant code example defines a doSomething() method in class Base that uses a private final lock, in accordance with CON04-J. Synchronize Use private final lock objects to synchronize classes that may interact with untrusted code using a private final lock object.

class Base {
  private final Object lock = new Object();

  public void doSomething() {
    synchronized (lock) {
      // ...	
    }
  }
}

class Derived extends Base {
  public void doSomething() {
    try {
      super.doSomething();
    } finally {
      logger.log(Level.FINE, "Did something"); 
    }
  }
}

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